A study was conducted to determine effects of grass carp (Ctenopharyngodon idella) on red swamp crawfish (Procambarus clarkii) in ponds in Louisiana. Grass carp averaging 190 g each, stocked 4 fish per 0.01-hectare pond, significantly reduced the average yield (number and total weight) of harvestable size (⩾74 nm) crawfish. Stomach analysis of carp suggests that competition for food (plant matter) was the cause of lowered crawfish yields. After vegetation was depleted, the amounts of animal matter (mainly cladocerans, crawfish, and gyrinids) in grass carp stomachs increased greatly.